Well pumping units are employed to pump fluids which are located beneath the surface of the earth to a position at or above the earth's surface. Access to the underground fluids is usually gained by drilling a hole through the earth's crust until the underground reservoir or deposit of fluid is reached. Then casing is lowered into the drill hole until the casing extends from the underground fluid reservoir up to the earth's surface. A pumping unit, including a polish rod and a sucker rod, is then employed with tubing located inside the casing to pump the fluids upward from the underground fluid reservoir to the earth's surface.
These well pumping units may be employed to pump any type of fluid, including water, natural gas, crude oil, etc. Since a common application of such well pumping units is to pump crude oil from an underground reservoir to the earth's surface, this invention will be discussed in that environment without limiting the scope of the invention solely to that environment.
As our crude oil resources become depleted, it becomes necessary to drill deeper for new crude oil sources and to pump this oil from a greater distance below the earth's surface and for long periods of time to accomplish what is known as secondary recovery from old wells. For secondary recovery, it is highly advantageous to provide a pumping unit with a long, slow stroke which results in longer pump and rod life and more efficient pumping, while limiting torque in the system and the size of its supporting structure. The deeper the well, the greater the force is needed to run conventional drive mechanisms and pumping units. Some of these conventional well pumping units are very inefficient when used in deep wells and may in fact be inoperable in very deep wells.
These well pumping units include a polish rod and a sucker rod within tubing which extends down through the well casing or pipe into the underground fluid reservoir. The sucker rod is attached to the lowermost end of the polish rod. The polish rod extends from the sucker rod to above the earth's surface. The polish and sucker rods are raised and lowered within the drill hole by a reciprocating drive mechanism. Usually, the reciprocating drive mechanism is connected to the polish rod by a cable. When the reciprocating drive mechanism moves in a first direction, it pulls the polish and sucker rods upward via the cable. When the drive mechanism moves in the second direction, it releases the cable and the gravity force on the polish and sucker rods takes up any slack in the cable and pulls the polish and sucker rods down the length of cable which has been released.
These well pumping units also include a derrick, boom or similar structure positioned above the drill hole and well pipe. This derrick typically supports the polish and sucker rods and any other apparatus associated therewith.
As stated above, these conventional well pumps have been unsatisfactory, particularly when employed with relatively deep wells, since the drive mechanisms when rotated in the first direction must have enough energy to lift the entire polish and sucker rod assembly and any liquid being pumped thereby. Then when the polish rod assembly is in the down cycle, the pumps rely solely on the force of gravity to drive the polish and sucker rods downward within the drill hole.
To overcome the problems associated with these pumping units of having the drive mechanism lift the entire weight of the polish and sucker rod assembly and associated fluids, well pumping units have been provided with a counterweight that somewhat offsets the weight of the polish and sucker rods so that the drive mechanism does not have to lift the entire weight of the polish and sucker rod assembly. In these systems, the counterweight is positioned so its weight opposes the weight of the polish and sucker rods as the polish and sucker rods are lifted. These counterweight well pumping units are of two basic types.
In the first type, a cable is attached to the top of the polish and sucker rods and extends to the drive mechanism. Also attached to the drive mechanism is a cable which connects the counterweight to the drive mechanism. The cables are attached to the drive mechanism such that the cables "pull" in opposite directions on the drive mechanism. The weight of the counterweight opposes the weight of the polish and sucker rods and fluid being pumped thereby when the polish rod assembly is being raised. Thus, the drive mechanism only has to raise the weight of the polish and sucker rods and associated fluid minus the weight of the counterweight. Examples of this type of pumping unit are disclosed in U.S. Pat. Nos. 2,370,029 to Gillespie; 2,681,623 to Kane; 2,683,424 to Kane; 3,285,081 to Kuhns et al; 3,528,305 to Kuhns et al; 3,695,117 to Ewing et al and 4,052,907 to Chardonneau et al.
The second general type of prior well pumping units with counterweights include the counterweight in the cable line between the polish rod and the drive mechanism. In these units, a cable extends from the top of the polish rod over a sheave located near the top of the derrick, to a counterweight and then to the drive mechanism. The counterweight is suspended on the opposite side of the sheave from the polish rod assembly so that the counterweight offsets the weight of the polish rod assembly. The weight of the polish and sucker rod assembly is partially negated by the counterweight before the weight is transmitted to the drive mechanism. Examples of this type of pumping unit are disclosed in U.S. Pat. Nos. 2,926,000 to Allen; 3,248,958 to Bender; 3,153,387 to Sadouet and 4,391,155 to Bender.
In these well pumping units, the drive mechanism must only provide enough energy to lift the weight differential between (1) the polish and sucker rod assemblies and fluids being pumped thereby and (2) the counterweight. However, when the polish rod assembly is in the down cycle, the counterweight opposes the downward movement of the polish rod assembly and reduces the gravitational pull forcing the polish rod assembly downward to the difference in the weight between the polish and sucker rod assembly and the counterweight.
The disadvantages of these prior well pumping units with counterweight systems are many. First, the polish and sucker rod assembly must always be heavier than the counterweight or else the entire system will fail since the units rely on the gravitational force on the polish and sucker rod assembly to drive these assemblies downward. If the counterweight is made equal in weight to or greater in weight than the polish and sucker rod, then the system will come to a complete standstill.
Second, the drive mechanism must still provide a force equal to the difference between the (1) polish and sucker rod assembly and associated fluid and (2) the counterweight. For deep wells, this difference in weight may be extremely high, making it necessary to have a very powerful drive mechanism.
For those embodiments which involve the counterweight on the opposite side of the drive mechanism from the polish rod assembly, these systems also include the disadvantage that all forces pull on the drive mechanism. This renders the drive mechanism less durable and it requires that the drive mechanisms be built to withstand large forces. The counterweight in these units does not directly balance the polish and sucker rod assembly, because all forces must be balanced through the drive mechanism.